Air momentum anemometer

ABSTRACT

A device for the measurement of horizontal wind velocity, especially low wind velocity. The horizontal component of momentum of the airstream is displaced from its normal flow. This is accomplished by either injecting an air jet into the airstream to displace the horizontal component of momentum, or withdrawing from the airstream the horizontal component of momentum by passing the airstream over an inlet into which inlet is drawn the horizontal component of the momentum flux of the airstream. The momentum of the air is converted to a force which is thus sensed and measured by a transducer, and since the force transduced is proportional to the first power of wind speed, large forces which may be accurately measured at low wind speeds are achieved.

United States Patent 1 Howell 5] March 6, 1973 AIR MOMENTUM ANEMOMETER[76] Inventor: Wallace E. Howell, P. O. Box 243,

Lexington, Mass. 02173 [22] Filed: April 5, 1971 [21] App]. No.: 131,120

[56] References Cited UNITED STATES PATENTS Li ...73/l94 M Boyle..73/202 Glasbrenner et al. ....73/228 South et a1. ..73/194 PrimaryExaminer-Richard C. Queisser Assistant ExaminerJohn P. BeauchampAttorney-Richard P. Crowley and Richard L. Stevens [5 7 ABSTRACT Adevice for the measurement of horizontal wind velocity, especially lowwind velocity. The horizontal component of momentum of the airstream isdisplaced from its normal flow. This is accomplished by either injectingan air jet into the airstream to displace the horizontal component ofmomentum, or withdrawing from the airstream the horizontal component ofmomentum by passing the airstream over an inlet into which inlet isdrawn the horizontal component of the momentum flux of the airstream.The momentum of the air is converted to a force which is thus sensed andmeasured by a transducer, and since the force transduced is proportionalto the first power of wind speed, large forces which may be accuratelymeasured at low wind speeds are achieved.

13 Claims, 5 Drawing Figures AIR STREAM PATENTEUHAR 619R 3.719.079

SHEET 2 BF 2 WALLACE E. HOWELL FIG. 5 BYMKMMJ ATTORNEYS AIR MOMENTUMANEMOMETER BACKGROUND OF THE INVENTION My invention is directed to adevice for the measurement of wind velocity, particularly velocities atvery low speeds. Presently available today are a number of types ofanemometers, but all are subject to deficiencies or shortcomings at lowwind speeds. One group of anemometers comprises cups or propeller-likemills that are moved by the wind at rates of speed roughly proportionalto the wind speed. These generally function well at moderate wind speedwhere the force causing the motion is large compared to the frictionalforces in the instruments themselves; but most anemometers of this typebecome unreliable at wind speeds of less than about 0.5 meters persecond and tend to stall at some speed not much lower than this. Asecond type of an anemometer is operated by the pressure of the wind ona pressure sensor of some sort. One type of anemometer or fluidmeasuring device wherein pressure differentials are measured is.exemplified by US. Pat. No. 3,343,413, South. Another type may besimilar to the air speed indicator of an airplane, or, in a primitiveform, may be nothing more complicated than a hanging plate blown awayfrom the vertical by the force of the wind. This type of device suffersfrom the fact that the pressure of the wind-is proportional to thesquare of its velocity and at very low velocities the square becomesvery small indeed. For example, a wind of centimeters per second exertsapressure of only 6 X 10' dynes per centimeters square. Thus it wouldhave to-impinge on the surface of about 1.7 square meters to exert aforce equivalent to the weight of 1 gram.

Still another type of anemometer widely used for very low wind speeds isthe hot wire or hot film type in which heat removal from a very finewire or filament increases with wind speed. These instruments havenotbecome particularly widespread in the field because of the mechanicalfragility and electrical complication; for instance, involved with thenecessity of taking the square root of the fluctuations in the hot wireresistance as a measure of wind pressure and the fourth root as ameasure of velocity; and interference from fog, rain, and snow. Still afurther anemometer depends upon the Doppler effect by which the soundwaves are propagated at the speed of sound plus the vector speed of windand so cover different distances and different directions in a giventime depending upon. the speed and direction of wind.

SUMMARY OF THE INVENTION My invention is directed to a device and methodwhich operates by sensing and measuring the momentum of air which unlikethe pressure varies linearly with wind velocity. The velocity of the airto be measured in the environment flowing along a path is displaced fromits travel along said path by a force acting substantially normal to thedirection of flow of the airstream.

The energy flux due to the motion of a stream ofair, equal to the massflux times half the square of the velocity, may be interpreted as adynamic pressure operating over the cross-section area of the stream, oralternatively as the momentum flux times half the velocity. My inventionis a device that transducesthe momentum flux of a sampled portion of theairstream into a force proportional to the wind velocity with atransducer for measuring the magnitude and direction 7 of this force.

The motion of. an air stream includes not only the velocity of stream asa whole but also its vorticity, so that vortical momentum flux isalso aproperty of the air stream. In this specification, the term velocityincludes the vortical as well as translational components of the'motionof the air stream and the term momentum or momentum flux includes thevortical as well as orthogonal components of momentum or momentum flux,and force includes the rotational as well as translational components ofthe net force.

The. momentum flux of a sample of the environmental air is'convertedinto a force proportional to that momentum exerted on an element of thedevice in such a manner that the force may be measured. The forcedevelopedis proportional to the first power of the wind velocity. Alldevices presently available in the state of theart that transduce forcesfrom the wind in order to measure the wind velocity produce forces thatvary as the square of thewind speed and hence produce very small forcesat very low windspeeds.

- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustrationof a preferred embodiment of my invention;

FIG. 2 is .a schematic illustration of an alternative embodiment of myinvention;

FIGS. 3 and 4 are schematic illustrations of the invention employed withhigh wind velocities; and

FIG. 5 is..a schematic illustration of the invention used to measure theair vorticity.

DESCRIPTION OF THE PREFERRED EMBODIMENT( S) Thev preferred embodiment ofthe invention is shown in FIG. 1 wherein a portion of the environmentalairstrearn, carrying'its proper momentum, is drawn into a .device 10from which it emerges with no horizontal momentum and thus exerts onthat device a force equal to the momentum flux of the indrawn air, insuch a manner that the force may be measured. Referring to FIG. 1, a fan12 or other suitable device draws air through an inlet. 14 and through ahoneycomb of vertical'passages 16 in the throat of the inlet so that theair emerges from the honeycomb with no horizontal momentum therein. Thehorizontal momentum flux of the indrawn. air is then exerted as ahorizontal force against the freely suspended honeycomb so that a windcauses thehoneycomb to be thrust in the downwind direction by a forceequal to the horizontal momentum flux of the indrawn air. Strain gagesl8 measure the com- :ponents of horizontal force on the honeycomb 16. Ifthe airstream has a horizontal velocity of 10 cm/sec and .times over theforce exerted by the wind on an object the size of such an inlet. Thisembodiment is particularly advantageous in that it includes the propertyof being an absolute instrument requiring no calibration; there is anexact equality, not simply proportionality, between the momentum. of theindrawn air and the force exerted on the inlet, and the latter istherefore an absolute measure of the former regardless of the geometricdetails of the device oi' of frictional losses in the airstream.

An alternative embodiment of the invention is shown in FIG. 2 wherein ajet of air from a source 20 is projected vertically into theenvironmental air or airstream moving horizontally passed it. The jetentrains a portion of the airstream in the wakestream of the jet so thatat a distance from the source 20 the wakestream contains both thevertical momentum delivered to it by its source and conservedthereafter, and the horizontal momentum of the entrained air. The coreof the wakestream becomes tilted at an angle, as shown by the arrows,the tangent of which is the ratio of the horizontal to the verticalmomenta, causing the wakestream core to follow a parabolic curve in thedownwind direction, that is to the right as shown in FIG. 2. The jet isdirected through a predetermined thickness, such as for example 35 cm,of the airstream from the source to impinge on a flat plate 22 engagedto vertical-force transducers 24 which in turn are secured to supports26. The plate 22 as shown is normal to the original direction of thejet. The momentum of the wakestream core is converted into a forceexerted on the plate 22, thecenter of the force being the locus of thejet core. Deflection of the core from its null position thereforegenerates a force moment about the null position equal to the jetmomentum times the length of its deflection from the null position. Ifthe jet momentum is dynes, and the wind speed is 10 cm/sec, for theexample of FIG. 2, the deflection of the core from its null positionwill be about 1 cm and the force moment 10 dynes-cm or about I gram-cm.This is about 200 times the force that the same wind speed would exerton a flat plate the size of the impact area of the wakestream, andrepresents a magnification of force through utilization of the momentumof the environmental air entrained into the wakestream as a means ofcontrolling the wakestream.

The invention, of course, may employ any type of transducer which wouldconvert the force acting on an element into a signal that may bemeasured; for example, a spring balance, a strain gage, or other devicethat will occur to persons skilled in instrumentation strain gaugeswhich may be used with the invention would in clude, for example, agauge such as manufactured by BLI-I, Inc., Waltham, Massachusetts, ModelNo.

Further, the particular design of FIG. 1 shows a cirflared. Also, otherdevices may be used for measuring the force exerted on the flat plate inthe second em- Since the honeycomb sensing element in the anemometersystem of FIG. 1 can be wholly enclosed in a housing, and since theforce exerted on it can be reduced by reducing the amount of air drawnthrough it, this anemometer lends itself to ruggedization for use inextremely severe wind situations such as hurricanes and tornadoes,making measurements in the wakestream of a jet engine, etc. For some ofthese applications, it is advisable to adopt some such arrangements asshown in FIGS. 3 and 4.

In FIG. 3 a honeycomb 40 on support 42 connected to strain gage system44 is protected by housing 46 that is firmly connected by stand-offs(not shown) to steel pipe 48 that serves as support and is connected tosuetion system (not shown) to draw air through the honeycomb.

In FIG. 4 air is drawn by motor and fan 50 upwardly through honeycomb 52which is mounted on force balance 54. Housing 56 is supported on opengrill of rods 58 from base 60.

The invention may also serve a further object, namely measuring thevorticity of the airstream. The vortical momentum of the air enteringthe-honeycomb exerts a torque on it that may be measured by a suitablyarranged strain gage system. Referring to FIG. 5, vorticity of airentering aperture 30 is converted by honeycomb 32 to a correspondingtorque sensed by torque sensor 34 supported on support 36. Fan and motor38 draw air through the honeycomb.- Sensor 34 may be combined withsensors of horizontal forces for use simultaneously as a regularanemometer.

Accordingly, in my invention a device has been provided wherein theforce transduced from a sample of the airstream is proportional to thefirst power of the wind speed and thus produces very large forces actingon the device at low wind speeds in comparison with the dynamic pressureof the wind. Having described my invention, what I new claim is: 1. Adevice for measuring wind velocity which comprises:

means to displace a portion of an airstream from its flow path in adirection substantially normal to said flow path; means to sense themomentum flux of the displaced portion of the airstream spaced apartfrom the means to displace the portion from the airstream in a directionsubstantially normal to the original flow path and to convert themomentum flux of the displaced portion of the airstream into a forcedirectly proportional to that displaced momentum flux; 1 meansresponsive to the means to sense and convert to produce a signalrepresenting said force; and means to measure the signal. 2. The deviceof claim 1 wherein the means to displace a portion of the airstreamincludes means to withdraw said portion from the airstream.

3. The device of claim 1 wherein the sensing and converting means is ahoneycomb-like device and the means to produce a signal responsive tothe force exject said jet stream in a direction normal to the means tosense and convert said momentum flux.

airstream which includes;

erted on the walls of the honeycomb includes a strain gauge.

4. The device of claim 1 wherein the means to dis- 5. The device ofclaim 4 which includes means to I direct said jet stream in a directionnormal to the flow of said airstream.

6. The device of claim 4 which includes means to in- 7. The device ofclaim 4 which includes means to in- 8. A device for measuring windvelocity which comprises:

means to withdraw a portion of an airstream from its flow path in adirection substantially normal to said flow path; 1

a honeycomb-like device axially aligned with and spaced apart from andintermediate the airstream and the means to withdraw a portion of theairstream from its flow path in a direction substantially normal to theoriginal flow path, the walls of the honeycomb defining cavitiestherein, to sense and convert the momentum flux of the displaced portionof the air stream such that substantially only the momentum flux of thedisplaced portion of the airstream in its direction of flow is sensedand converted into a force directly proportional to that displacedmomentum flux;

means responsive to the honeycomb-like device to measure the forceexerted on the walls of the honeycomb secured to said honeycomb toproduce an electrical signal representing said force; and

means to measure the signal.

9. A method for measuring the velocity of an displacing at least aportion of an airstream in a direction substantially normal to the flowpath of said air stream;

sensing the momentum flux of the displaced portion of the airstream at alocation spaced apart from the airstream displacing location in adirection substantially normal to the original flow path of theairstream and converting the momentum flux of the displaced portion ofthe airstream into a force directly proportional to that displacedmomentum flux;

producing a signal corresponding to the force; and

measuring the signal so produced.

10. The method of claim 9 wherein the displacing of the airstreamcomprises injecting an air jet into the stream in a directionsubstantially normal to the flow of the airstream to displace a portionof the airstream.

11. The method of claim 9 wherein the displacing of the airstreamcomprises injecting normally into the airstream an air jet;

entraining in the wakestream of the air jet the horizontal component ofmomentum flux of the portion of the airstream whereby at a distance fromits source the wakestream contains both the yertical momentum flux ofthe air jet delivered to it at the air et source and the horizontalmomentum flux of the entrained environmental air; and

measuring the force moment generated by deflection of the core of thewakestream from its null position.

12. The method of claim 9 wherein the displacing of the airstreamcomprises withdrawing the portion of the airstream from the airstream;

exerting a deflecting force on a previously positioned transducer withthe horizontal component of momentum flux of said withdrawn portion; andmeasuring the amount of deflecting force exerted on said transducer.

13. The method of claim 9 wherein the displaced momentum flux is avortical momentum flux and the force measured is a torque.

1. A device for measuring wind velocity which comprises: means todisplace a portion of an airstream from its flow path in a directionsubstantially normal to said flow path; means to sense the momentum fluxof the displaced portion of the airstream spaced apart from the means todisplace the portion from the airstream in a direction substantiallynormal to the original flow path and to convert the momentum flux of thedisplaced portion of the airstream into a force directly proportional tothat displaced momentum flux; means responsive to the means to sense andconvert to produce a signal representing said force; and means tomeasure the signal.
 1. A device for measuring wind velocity whichcomprises: means to displace a portion of an airstream from its flowpath in a direction substantially normal to said flow path; means tosense the momentum flux of the displaced portion of the airstream spacedapart from the means to displace the portion from the airstream in adirection substantially normal to the original flow path and to convertthe momentum flux of the displaced portion of the airstream into a forcedirectly proportional to that displaced momentum flux; means responsiveto the means to sense and convert to produce a signal representing saidforce; and means to measure the signal.
 2. The device of claim 1 whereinthe means to displace a portion of the airstream includes means towithdraw said portion from the airstream.
 3. The device of claim 1wherein the sensing and converting means is a honeycomb-like device andthe means to produce a signal responsive to the force exerted on thewalls of the honeycomb includes a strain gauge.
 4. The device of claim 1wherein the means to displace a portion of the airstream includes a jetstream injected into said airstream.
 5. The device of claim 4 whichincludes means to direct said jet stream in a direction normal to theflow of said airstream.
 6. The device of claim 4 which includes means toinject said jet stream in a direction normal to the means to sense andconvert said momentum flux.
 7. The device of claim 4 which includesmeans to introduce the jet stream in a direction normal to the flow ofthe airstream to the means and sense and convert momentum flux.
 8. Adevice for measuring wind velocity which comprises: means to withdraw aportion of an airstream from its flow path in a direction substantiallynormal to said flow path; a honeycomb-like device axially aligned withand spaced apart from and intermediate the airstream and the means towithdraw a portion of the airstream from its flow path in a directionsubstantially normal to the original flow path, the walls of thehoneycomb defining cavities therein, to sense and convert the momentumflux of the displaced portion of the air stream such that substantiallyonly the momentum flux of the displaced portion of the airstream in itsdirection of flow is sensed and converted into a force directlyproportional to that displaced momentum flux; means responsive to thehoneycomb-like device to measure the force exerted on the walls of thehoneycomb secured to said honeycomb to produce an electrical signalrepresenting said force; and means to measure the signal.
 9. A methodfor measuring the velocity of An airstream which includes; displacing atleast a portion of an airstream in a direction substantially normal tothe flow path of said air stream; sensing the momentum flux of thedisplaced portion of the airstream at a location spaced apart from theairstream displacing location in a direction substantially normal to theoriginal flow path of the airstream and converting the momentum flux ofthe displaced portion of the airstream into a force directlyproportional to that displaced momentum flux; producing a signalcorresponding to the force; and measuring the signal so produced. 10.The method of claim 9 wherein the displacing of the airstream comprisesinjecting an air jet into the stream in a direction substantially normalto the flow of the airstream to displace a portion of the airstream. 11.The method of claim 9 wherein the displacing of the airstream comprisesinjecting normally into the airstream an air jet; entraining in thewakestream of the air jet the horizontal component of momentum flux ofthe portion of the airstream whereby at a distance from its source thewakestream contains both the vertical momentum flux of the air jetdelivered to it at the air jet source and the horizontal momentum fluxof the entrained environmental air; and measuring the force momentgenerated by deflection of the core of the wakestream from its nullposition.
 12. The method of claim 9 wherein the displacing of theairstream comprises withdrawing the portion of the airstream from theairstream; exerting a deflecting force on a previously positionedtransducer with the horizontal component of momentum flux of saidwithdrawn portion; and measuring the amount of deflecting force exertedon said transducer.